The present invention relates to a method and apparatus for controlling the amount of exposure of a photosensitive substrate in an exposure apparatus used in a lithography process for producing, for example, semiconductor devices, liquid crystal display devices, image pickup devices (such as charge coupled devices), or thin-film magnetic heads. The invention is suitable for use in full wafer exposure apparatus and, more particularly, for use in exposure control in a scanning projection exposure apparatus of, for example, a scan-and-step type.
The production of semiconductor devices and the like conventionally employs a projection exposure apparatus to project and transfer the pattern of a reticle onto each shot area on a wafer (or a glass plate) coated with a photoresist. As a basic function, the projection exposure apparatus controls the total amount of exposure (total exposure energy) of each point in each shot area on a wafer within an appropriate amount range.
For the exposure control of conventional full wafer projection exposure apparatus, such as steppers, cut-off control is normally performed whether the exposure light source employed is a continuous light source such as an extra-high pressure mercury lamp, or a pulsed laser light source such as an excimer laser light source. In the cut-off control, a portion of the light for exposure of a wafer coated with a photosensitive material is branched and directed to an integrator sensor formed of a photoelectric detector, whereby the amount of exposure of the wafer is indirectly detected. The light emission is continued until the total of exposure amounts detected by the integrator sensor exceeds a predetermined level (critical level) corresponding to a total amount of exposure (hereinafter, referred to as xe2x80x9cset amount of exposurexe2x80x9d) that is required for the photosensitive material used (in the case of continuous light, a shutter is closed when the critical level is exceeded).
In the case of exposure using a pulsed laser light source as an exposure light source, a desired precision reproducibility in exposure control can be achieved by using at least a certain number of laser light pulses for exposure (hereinafter, the xe2x80x9ccertain numberxe2x80x9d will be referred to as xe2x80x9cminimum exposure pulse numberxe2x80x9d) because energy varies from one laser light pulse to another. In the case of a highly sensitive resist for which the set amount of exposure is small, the exposure to at least the minimum exposure pulse number of laser light pulses may become impossible if laser light from the pulsed laser source is directly used for exposure. Therefore, if the set amount of exposure is small, the pulsed laser light must be reduced in intensity by, for example, a light reducing device disposed in the optical path, so that at least the minimum exposure pulse number of laser light pulses can be employed for exposure.
To enable transfer of a pattern of an increased area to a wafer with a high precision without imposing severe requirements on the projection optical system, a step-and-scan projection exposure apparatus has recently been developed that synchronously scans or moves a reticle and a wafer relative to the projection optical system while projecting the images of portions of the pattern of the reticle onto the wafer using the projection optical system, so as to sequentially transfer portions of the pattern of the reticle onto individual shot areas on the wafer by exposure. In such a scanning exposure type apparatus, exposure control regarding a point on a wafer is impossible, and the aforementioned cut-off control cannot be applied. Therefore, for exposure control in scanning exposure type exposure apparatus, the conventional art normally employs a method (open exposure control method) that controls the amount of exposure simply by totalling the light quantity of each illumination pulse, or a method (every pulse exposure amount control method) that controls the energy of every illumination light pulse by measuring the total amount of exposure of an area on a wafer in real time, the area included in a slit-like illumination field (exposed area) extending in the scanning direction, and calculating a target energy value of the next illumination light pulse based on the total amount of exposure.
The former control method requires fine adjustment of pulse energy to satisfy the following equation (1) wherein the number of exposure pulses is an integer in order to achieve a desired linearity in the exposure control:
(set amount of exposure)=(number of pulses)xc3x97(average energy per pulse)xe2x80x83xe2x80x83(1)
In equation (1), the average energy per pulse is a value measured by an integrator sensor immediately before exposure. The latter control method requires fine adjustment of the pulse energy at every emission of a pulse.
A conventional pulsed laser light source used in connection with either of the control methods contains an energy monitor formed by a photoelectric detector, and performs feedback control of the laser electric power source so that the detection result by the energy monitor conforms to an output energy value (central energy value) provided by an external device in order to constantly output light pulses of the same amount of energy. More specifically, the central energy value inputted to the pulsed laser light source is fixed, and the fine modulation of energy of a light pulse is performed using an energy fine modulator.
FIG. 8(a) illustrates a conventional energy fine modulator of a double grating type. In this fine modulator, a stationary grating 41 having light transmitting portions and blocking portions formed at a predetermined pitch and another grating 42 movable in the direction of the grating pitch are arranged one over the other in the optical path of a laser beam LB emitted in a pulsed manner. By shifting the positions of the gratings 41, 42 relative to each other, the laser beam transmittance can be finely modulated. FIG. 8(b) illustrates another conventional energy fine modulator in which two glass plates 43, 44 each coated with anti-reflection coating on both sides are arranged in the optical path of a laser beam LB, with the glass plates symmetrically inclined at a variable inclination angle xcex8. Exploiting a property of the glass plates 43, 44 that the transmittance varies depending on the incident angle of the laser beam LB, the fine modulator finely adjusts the overall laser beam transmittance by controlling the inclination angle xcex8.
The conventional energy fine modulators as shown in FIGS. 8(a) and 8(b), however, have drawbacks in that since a mechanical drive is employed for the adjustment of transmittance, high-speed adjustment of transmittance is difficult. Moreover, since neither of the fine modulators is able to achieve a maximum transmittance of 100%, an energy loss results even in an initial maximum transmittance state, thus adversely affecting device efficiency in utilizing the pulsed light.
The double grating type energy fine modulator as shown in FIG. 8(a) has another drawback in that even though the fine modulator is disposed at the light source-side of a fly eye lens provided as an optical integrator, the overlapping effect of the fly eye lens becomes small if the aperture of the illumination diaphragm is small, so that the grating pattern slightly remains as an illumination non-uniformity in an image. A small aperture of the illumination diaphragm means that a so-called a value, that is, a coherence factor, is small.
Accordingly, it is an object of the present invention to provide an exposure control method capable of finely modulating the energy of illumination light and therefore the amount of exposure of a photosensitive substrate at a high speed without inserting a mechanically-driven energy fine modulator for finely modulating the transmittance (light reduction rate) in the optical path of illumination light, and without causing an energy loss along the optical path of illumination light.
It is another object of the present invention to provide an exposure control apparatus capable of employing the exposure control method of the invention.
According to an aspect of the present invention, there is provided an exposure control method for controlling an amount of exposure of a photosensitive substrate to illumination light in an exposure apparatus that illuminates a mask having a pattern with illumination light emitted from an exposure light source and thereby exposes a photosensitive substrate to the illumination light through the mask to transfer the pattern of the mask to the substrate. In the method, the emitting power of the exposure light source is finely modulated within a predetermined range to control the amount of exposure of the photosensitive substrate. By directly controlling the emitting power of the exposure light source, the method of the invention finely modulates the energy of illumination light without employing mechanical drive and without causing an energy loss along the optical path of the illumination light.
If a pulsed laser light source, such as an excimer laser light source, is used as the exposure light source, the range of energy fine modulation required in order to obtain an integer number of exposure light pulses for each point on the photosensitive substrate is at most plus or minus several percent. Therefore, if an excimer laser light source is used, it becomes possible to make the central energy (emission power) of each pulse from the light source variable within the range required for an integer number of exposure light pulses.
It is preferred that the illuminance of the illumination light in each shot area on the photosensitive substrate be monitored during sequential exposures of a plurality of shot areas, and that the emission power of the exposure light source be finely modulated in accordance with a monitoring deviation result from a target value for exposure of the next shot area on the photosensitive substrate. Thereby, the emission power setting of the exposure light source can be updated, for example, during a shift from one shot area to another.
According to another aspect of the present invention, there is provided an exposure control method for controlling an exposure amount of a photosensitive substrate to illumination light in an exposure apparatus that illuminates a mask having a pattern with illumination light emitted from an exposure light source and thereby exposes a photosensitive substrate to the illumination light through the mask to transfer the pattern of the mask to the substrate. The method includes determining a table of correlation between an illuminance of the illumination light occurring inside or immediately out of the exposure light source and an illuminance of the illumination light occurring on the photosensitive substrate, and controlling the amount of exposure of the photosensitive substrate to the illumination light on the basis of the table of correlation.
According to this method, the illuminance of the illumination light at the exposure light source is detected by, for example, an energy monitor disposed in the exposure light source, and the illuminance of the illumination light on the photosensitive substrate is detected by, for example, an energy monitor of the exposure apparatus. The table of correlation between the outputs from the two energy monitors is determined and stored as a control table. The amount of exposure of the photosensitive substrate is controlled by, for example, fine modulation of the exposure light source emission power. This fine modulation is based on the output from the energy monitor of the exposure apparatus, not the output from the energy monitor of the exposure light source. More specifically, an output of the energy monitor in the exposure light source is determined from the output of the energy monitor of the exposure apparatus and the table of correlation, and the thus-determined output of the energy monitor in the exposure light source is used as a reference for finely modulating the emission power of the exposure light source. Thereby, the exposure control linearity (that is, linearly between target amount of exposure and actual amount of exposure) can be obtained on the basis of the output of the energy monitor of the exposure apparatus.
It is preferred to determine the table of correlation when dummy light emission from the exposure light source is performed without exposing the photosensitive substrate.
It is also preferred to update the table of correlation if a change is expected in the correlation between the illuminance of the illumination light occurring inside or immediately out of the exposure light source and the illuminance of the illumination light occurring on the photosensitive substrate.
Since the correlation between the outputs of the two energy monitors is expected to fluctuate over time, an exposure apparatus-side energy monitor-based exposure control linearity that is stable for a long time can be achieved by sampling data for preparation of a correlation table to update the correlation table if there is a likelihood that the deviation from the correlation will exceed a predetermined amount, i.e., a likelihood that the error of the correlation table will exceed an allowable amount.
In addition, if the exposure light source is a pulsed laser light source, there exists a requirement for a desired exposure reproducibility (reproducibility of the amount of exposure for each area on the photosensitive substrate in the case of scanning exposure), i.e., a minimum number of pulses required (minimum exposure pulse number Nmin). To meet this requirement, the light intensity is reduced by, for example, an energy rough modulator disposed in the optical path, depending on the relationship between the set amount of exposure and the energy per pulse, so that the exposure will be completed by using at least Nmin number of exposure pulses. The range of pulse energy incident on the energy monitor in the exposure light source is narrow, whereas the range of pulse energy incident on the exposure apparatus-side energy monitor is wide. The difference therebetween is, for example, at least 100 fold if an excimer laser light source is employed. Since the correlation between the two energy monitors changes depending on the transmittance in the energy rough modulator, it is preferred to sample data for a correlation table, as well as for an energy determination, prior to exposure while the energy rough modulator is in operation, and to prepare a correlation table based on the sampled data for updating the correlation table. By performing this process, the exposure control linearity based on the exposure apparatus-side energy monitor can be achieved for any set amount of exposure.
If the exposure light source is a pulsed emission type light source in either of the two exposure control methods, it is preferred that the exposure light source pre-emit a number of pulses necessary for stabilization of the emission power of the exposure light source when the light source emission power setting is changed. In a scanning type exposure apparatus, pre-emission (dummy emission) of light may be performed while the illumination field aperture is closed, for example, during the period of acceleration of a stage system, i.e., the preparation time for the scanning exposure operation, or the stabilizing time.
According to still another aspect of the present invention, there is provided an exposure control apparatus for controlling an amount of exposure of a photosensitive substrate to illumination light in an exposure apparatus that illuminates a mask having a pattern with illumination light emitted from an exposure light source and thereby exposes a photosensitive substrate to the illumination light through the mask to transfer the pattern of the mask to the substrate. The apparatus includes a light source modulation unit that finely modulates an emission power of the exposure light source within a predetermined range, a first energy monitor for detecting an illuminance of the illumination light occurring inside or immediately out of the exposure light source, a second energy monitor for detecting an illuminance of the illumination light on the photosensitive substrate, and a controller for controlling the light source modulation unit on the basis of the detection results of the first and second energy monitors. This exposure control apparatus is able to employ either of the exposure control methods.